The invention relates to an on chip current source for converting a high voltage input into an output current for supplying low voltage circuitry said current source comprising a MOS transistor having a first terminal connected to a high voltage input, a second terminal in use providing an output current to a current output terminal and a gate, said current source further including bias current generating means connected to said high voltage input and arranged for generating in use a small bias current for said gate, said gate further being connected to an output of a voltage comparator means, said voltage comparator means, said voltage comparator means being arranged for comparing a sensed voltage caused by said output current over a voltage sensor means with a reference voltage and providing at said comparator output a regulation voltage for said gate based on the difference between said sensed voltage and said reference voltage.
A current source of the type described above is traditionally used as an on chip current source for supplying low voltage circuitry from a high voltage source. A drawback of those prior art current sources is their relatively low output impedance. Another drawback is that at a maximum output voltage the output current is reduced to zero.
U.S. Pat. No. 5,815,383 discloses a switchmode power supply (SMPS) high voltage start-up integrated circuit (IC) with output voltage sensing and output current limiting. The high voltage start-up IC allows low voltage pulse width modulated (PWM) controller ICs to operate directly off rectified AC lines of up to 450 VDC. The high voltage start-up IC allows PWM controller ICs to start-up with a start threshold larger than its operating voltage. The output voltage is monitored internally so that the internal high voltage switch turns off when the output voltage decreases below an internally set trip point voltage (Voff). The internal high voltage switch remains off and an external auxiliary voltage is generated and applied to the output voltage. If the output voltage falls below a lower set trip point voltage (Vreset) the internal high voltage switch turns back on. This allows the start-up circuit to reset itself when the PWM controller IC""s auxiliary voltage does not power up properly.
It is, inter alia, an object of the invention to provide an improved current source for on chip use having an increased output impedance and/or an increased output voltage range as compared to the traditional circuitry. To this end, the invention provides a current source as defined in the independent claims. Advantageous embodiments are defined in the dependent claims.